What happens when two galaxies collide?
A. On the largest scales, the changing gravitational fields cause the galaxies to distort their shapes tremendously to produce great streams of stars and gas that are often ripped from each of the galaxies and hurled into intergalactic space. Most of the matter, however, settles back into a new system which often looks nothing like either of the galaxies before the event. When the interstellar clouds in each of the galaxies collide, they can trigger bursts of star formation resulting in very massive, short lived, stars being formed. These stars form in large numbers and over small enough regions their light can combine to turn the galactic collision into a so-called star-burst system. If the cores have massive black holes, the systems can flare-up into quasar, or near-quasar brilliance for millions of years. For individual stars, they are so small compared to their average distances that you might get only a handful of close encounters or impacts out of literally hundreds of billions o
